Y型微通道内气泡生成过程及行为研究

为了详细研究在挤压机制和剪切机制作用下,气液两相流在Y型微通道内的流动过程,文中利用数值模拟的方法,采用VOF模型对二维Y型微通道内气泡断裂进行了模拟。模拟结果表明:随着连续相速度的增加,液柱长度和气泡的生成周期逐渐减小;气泡的破裂受到压差和涡流的共同作用;在低连续相速度下,气泡的断裂主要受到挤压机制作用,在高连续相速度下,气泡断裂受到剪切机制的影响。这为进一步优化微流控系统和微通道换热器提供了理论基础。     

微通道液-液两相流动特性实验研究

利用高速摄像机与Canny算法,以硅油为离散相,含0. 5%SDS的蒸馏水为连续相,研究了凹穴型微通道内液-液两相流动特性。结果表明,直通道内观察弹状流、过渡流、滴状流3种流型。随着毛细数的增大,液滴形成机理由挤压机制向剪切机制转变,液滴速度逐渐增大,液滴长度逐渐减小。随着连续相流量的增加,液滴形成时间逐渐减小,且挤压机制生成液滴的时间大于剪切机制。凹穴结构减弱了壁面对液滴的限制,液滴速度降低,T型交汇处压差降低,相同工况下的液滴尺寸大于对冲T型微通道的液滴尺寸。     

十字交叉微通道内微液滴生成过程的数值模拟

采用VOF模型对十字交叉微通道内微液滴的生成进行三维数值模拟,获得了拉伸挤压、滴状剪切、单分散射流等单分散微液滴的生成机制以及紊乱射流、节状形变流、管状流和滑移流等两相流型,模拟与实验结果相吻合验证了模拟的有效性。液液两相流型主要受两相流速、两相界面张力以及连续相黏度的影响,发现随着连续相的流量增大,微液滴的生成尺寸减小,生成频率增大;而离散相流量的影响则相反。两相表面张力与连续相黏度分别在低连续相Ca数和高连续相Ca数条件下分别起主导作用。在低连续相Ca数(U_d<0.03 m·s^-1)的拉伸挤压和滴状剪切流流型下,微液滴生成尺寸随着表面张力系数的减小而减小,在射流条件下反而增大,微液滴的生成频率变化则相反。在高连续相Ca数(U_d>0.03 m·s^-1)下,微液滴的生成尺寸随着连续相黏度的增大而减小,微液滴的生成频率变化则相反。另外,壁面接触角在拉伸挤压流型下对微液滴生成无太大影响,但在滴状剪切和单分散射流流型下,接触角减小会导致微液滴无法稳定生成。     

并行微通道内液液两相流及介尺度效应

使用高速摄像仪研究了T形并行微通道液液两相流的流型。以甘油-水溶液为分散相、含5%（质量分数）道康宁的硅油为连续相,下游通道中观察到了塞状流、液滴流、环状流和并行流4种流型,绘制了流型图及流型转变线。研究了后空腔中液滴群的形态,运用介尺度概念分析了后空腔中液滴群的行为对流量分配的影响。观察到后空腔中液滴群的挤压、松散、有序排列和并行排列等4种形态,不同形态的转变主要受两相流量比的控制。研究了两相流量比对并行微通道内流量分配的影响,以及分析了不同操作条件下影响流量分配的主导因素。在两相流量比较小时,流量分配由下游通道的流体阻力主导,而两相流量比较大时由后空腔内液滴群动力学主导。     

聚焦十字型微通道内高黏流体中气泡生成动力学

利用高速摄像仪观察了聚焦十字型微通道内高黏度（630 mPa·s）的甘油-水溶液中氮气气泡的生成过程。研究了气泡生成过程以及气泡体积和最小颈部半径的变化规律。结果表明,高黏流体内气泡生成过程可分为回缩、膨胀、挤压和最终破裂4个阶段。气泡体积在膨胀和挤压阶段均随时间线性增长,但挤压阶段的斜率大于膨胀阶段的斜率。气泡最小颈部半径随时间变化分为两个不同的阶段：在挤压阶段,颈部半径随剩余时间呈幂率关系;而在最终破裂阶段,颈部半径随时间呈线性关系。     

微通道内浆料体系中的气泡生成特性及尺寸预测

利用高速摄像仪对T型微通道内浆料体系中的气泡生成频率和气泡尺寸进行了研究。以氮气作为分散相,含0.35%（质量分数）表面活性剂（SDS）不同浓度玻璃珠的甘油-水溶液为连续相。实验考察了弹状流下气液两相流量、颗粒浓度以及浆料表观黏度对气泡生成频率及气泡尺寸的影响。结果表明：在弹状流下,当分散相流量一定时,随着连续相流量的增大,气泡的生成频率增大而气泡尺寸减小。当连续相流量一定时,随着分散相流量的增大,气泡生成频率和气泡尺寸均增大。随着颗粒浓度的增大,浆料的表面张力减小,表观黏度增大,气泡生成频率增大而气泡尺寸减小。提出了T型微通道内浆料体系中生成气泡尺寸的预测模型,模型具有良好的预测精度。     

液滴内空泡生长控制机理及影响因素的研究

在超空化燃油射流使得喷雾中部分燃油分裂液滴内含有空化气泡;空化气泡的生长对液滴的分裂与雾化具有重要的影响。研究基于VOF方法对燃油液滴内空化气泡的生长过程进行了数值模拟,结合R-P方程对单液滴内空化气泡生长控制机理及影响因素进行了分析。结果表明,单液滴内空化气泡的生长可以按控制机理划分为表面张力控制阶段、综合竞争阶段和惯性力控制阶段。在第I生长阶段,空泡的生长主要受表面张力的控制作用;在第II生长阶段,空泡的生长主要受表面张力、惯性力及黏性力三者的综合作用;在第III生长阶段,空泡的生长主要受惯性力的控制作用。最后,利用建立的数值计算模型对表面张力系数、液体黏度及液体密度对液滴内气泡生长过程的影响进行了分析。     

T型微通道内浆料体系中气泡的生成动力学

实验研究了T型微通道内浆料体系中弹状气泡的生成动力学,重点考察了颗粒粒径的影响。气泡的生成过程可划分为四个阶段：填充阶段、挤压阶段、过渡阶段和快速夹断阶段。在填充阶段、挤压阶段和快速夹断阶段,气泡最小颈部宽度与时间呈幂律关系。在过渡阶段,气泡最小颈部宽度与时间呈线性关系。浆料体系中气泡的挤压阶段和过渡阶段随颗粒粒径的减小而缩短。连续相流量及颗粒粒径对填充阶段幂律指数无显著影响;挤压阶段和快速夹断阶段的幂律指数以及过渡阶段的线性斜率均随颗粒粒径的增大而减小,随连续相流量的增大而增大。     

变截面通道内流动聚焦微液滴生成数值研究

化学分析研究的不断深入,对液滴微流控精度提出了更高的要求。设计了不同孔径尺寸的流动聚焦通道模型,模拟油-水两相流动剪切微液滴生成过程。采用Level Set方法处理两相流动界面,研究了流动聚焦模型下通道尺寸和油-水两相流量比对生成液滴尺寸的影响。研究结果对实现微液滴生成过程的精准操控具有一定的参考意义。     

正弦型微通道内液-液两相流型及流动特性实验研究

采用实验的方法对不混溶的液液两相流体在不同入口结构下的正弦微通道(直通道正弦、波峰正弦和波中正弦)内液滴的流动特性进行了分析。硅油作为离散相,含有0.5% SDS的蒸馏水作为连续相,观测到弹状流、滴状流和射状流。分析了两相流动参数及不同的微通道入口结构对流型和液滴长度的影响。流型受微通道入口结构影响较大,波峰正弦微通道能够生成最大范围的稳定的流型。液滴长度随离散相体积流量和离散相与连续相体积流量之比的增大而增大,随连续相的体积流量和毛细数的增大而降低。微通道入口结构对液滴长度有影响,直通道的正弦微通道内液滴长度最短,更有利于液滴的形成。三种通道生成的液滴中,最大的液滴尺寸是最小的液滴尺寸的1.15~1.39倍,但正弦流动段对液滴速度几乎没有影响。     

Squeezing-to-dripping transition for bubble formation in a microfluidic T-junction

The aim of this paper is to investigate the squeezing-to-dripping transition for bubble formation in a microfluidic T-junction by cross-flowing rupture technique using a high-speed digital camera. Experiments were conducted in a glass microfluidic T-junction with the cross-section of the microchannel of 120 μm wide and 40 μm deep. N₂ bubbles were generated in glycerol–water mixtures with several concentrations of surfactant sodium dodecyl sulfate (SDS). Three different regimes were identified for generating different kinds of bubbles: squeezing, dripping and transition regimes. Various forces exerted on the gaseous thread in different regimes were analyzed. Long slug bubbles were formed in the squeezing regime, while dispersed bubbles in the dripping regime. The transition regime formed short slug bubbles. The bubble sizes in various regimes could be correlated with several dimensionless numbers such as the ratio of gas/liquid flow rates and capillary number. The two-step model for droplets (Steegmans et al., 2009) was extended to describe the bubble formation.     

Breakup dynamics for high‐viscosity droplet formation in a flow‐focusing device: Symmetrical and asymmetrical ruptures

The breakup mechanism of high‐viscosity thread for droplet formation in a flow‐focusing device is investigated using a high‐speed digital camera. Aqueous solution of 89.5%‐glycerol is used as the dispersed phase, while silicone oil as the continuous phase. The breakup process of the dispersed thread presents two categories: symmetrical rupture and asymmetrical rupture. Furthermore, the rupture behavior could be divided into two stages: the squeezing stage controlled by the squeezing pressure and the pinch‐off stage controlled by viscous stresses of both phases and surface tension. Specifically, it suggests that the differences in the shape of the liquid–liquid interface and the dynamics in the two breakup processes are caused by the disparity of the strain field at the point of detachment. Moreover, the thinning rate and the dynamics of the dispersed thread change with the viscosity of the continuous phase, but are less dependent of the flow rate of the continuous phase. © 2015 American Institute of Chemical Engineers AIChE J, 62: 325–337, 2016     

阶梯式T型微通道内液滴、气泡分散规律

采用高速摄像仪对嵌入毛细管的阶梯式T型微通道内液滴和气泡的分散规律进行研究。考察了两相流量、黏度、表面活性剂浓度等因素对分散流型及分散尺寸的影响规律。结果表明,对于液滴分散过程,表面活性剂的浓度和连续相流量决定了分散流型,随二者增大,流型从dripping流向jetting流转变。对于气泡分散过程,实验范围内仅存在squeezing、dripping流型,表面活性剂的加入对气泡分散过程影响可忽略。嵌入毛细管的阶梯式T型微通道内获得的液滴、气泡直径小于微通道直径,根据实验结果基于两相流量和毛细管数分别建立了计算液滴、气泡分散尺寸的半经验模型,模型与实验结果符合良好。     

Bubble/droplet formation and mass transfer during gas–liquid–liquid segmented flow with soluble gas in a microchannel

Microchannels have great potential in intensification of gas–liquid–liquid reactions involving reacting gases, such as hydrogenation. This work uses CO₂–octane–water system to model the hydrodynamics and mass transfer of such systems in a microchannel with double T‐junctions. Segmented flows are generated with three inlet sequences and the size laws of dispersed phases are obtained. Three generation mechanisms of dispersed gas bubbles/water droplets are identified: squeezing by the oil phase, cutting by the droplet/bubble, cutting by the water–oil/gas–oil interface. Based on the gas dissolution rate, the mass transfer coefficients are calculated. It is found that water droplet can significantly enhance the transfer of CO₂ into the oil phase initially. When bubble‐droplet cluster are formed downstream the microchannel, droplet will retard the mass transfer. Other characteristics such as phase hold‐up, bubble velocity and bubble dissolution rate are also discussed. The information is beneficial for microreactor design when applying three‐phase reactions. © 2016 American Institute of Chemical Engineers AIChE J, 63: 1727–1739, 2017     

微通道内液滴/气泡破裂动力学分析

微化学工程与技术是现代化学工程学科的前沿领域。微通道内液滴及气泡破裂动力学是决定多相过程并行微通道数目放大的基础与难点。破裂流型转换条件、界面动力学和尺寸调控等三方面是微通道内液滴与气泡破裂动力学的主要研究对象。讨论了对称微通道、非对称微通道、多级微通道、旁路微通道、含有障碍物的微通道内气泡和液滴破裂行为及影响因素,指出了目前微尺度下气泡与液滴破裂行为相关研究工作存在的不足,并对该领域未来的发展进行了展望。     

Formation characteristics of Taylor bubbles in a T-junction microchannel with chemical absorption

This study focuses on the effect of chemical absorption on the formation dynamic characteristics and initial length of Taylor bubbles. The temporal evolutions of neck width and length of gaseous thread and initial length with and without chemical absorption were investigated with the Capillary number and Hatta number between 0.0010–0.0073 and 1.8–5.8 respectively. The squeezing regime with typical three stages, expansion, squeezing and pinch off is observed for both two processes. Compared with the non-absorption process, the increase of formation time in the chemical absorption process arises mainly from the expansion stage, and the decrease of initial length is from the necking stage. In addition, the temporal length evolution satisfies the power-law scale with the same exponent but a smaller pre-exponential factor. The correlations of neck width for stage transition and initial length with Hatta number demonstrate the enhancement effect of chemical absorption on bubble formation dynamics and initial length at relatively high chemical reaction rates and long formation time. This study provides insight into the bubble formation mechanism and helps to regulate the bubble initial size with chemical absorption.     

Formation dynamics and size prediction of bubbles for slurry system in T-shape microchannel

The bubble formation dynamics and size manipulation in the slurry of polystyrene microspheres in the microfluidic T-junction were visually investigated by a high-speed camera. Based on the evolution of the bubble neck with time, the formation process of bubbles is divided into three stages: filling, squeezing and pinch-off. The particle concentration has an obvious effect on the squeezing stage, while less impact on the filling and pinch-off stages. In the squeezing stage, the evolution of the dimensionless minimum neck width of bubbles with time could be described by a power-law relationship. The increase of the particle concentration or continuous phase flow rate could lead to the increase of body flow of the continuous phase and the enhancement of the squeezing force acted on the bubble neck, correspondingly, the power-law index α in the squeezing stage enlarges. Moreover, the bubble size increases with the increase of the gas phase flow rate and the decrease of the particle concentration and continuous phase flow rate. However, the effect of the particle concentration on the bubble size weakens with the increase of the continuous phase flow rate. In addition, a new prediction correlation of the bubble size for the slurry system in a T-shape microchannel was proposed with good prediction accuracy.     

T型微通道内浆料体系中气泡生成行为与尺寸预测

实验研究了T型微通道内浆料中气泡的生成过程和尺寸。聚丙乙烯微球浆料和N₂分别为连续相和分散相。气泡的生成过程可分为三个阶段：膨胀阶段、挤压阶段和快速夹断阶段。随着浆料浓度的增大,膨胀阶段时长几乎没有变化,挤压阶段显著缩短,而快速夹断阶段略有缩短。在膨胀阶段和快速夹断阶段,气泡颈部宽度与无量纲剩余时间均呈幂率关系,而挤压阶段气泡颈部宽度与时间呈线性关系。考察了浆料浓度、气相和浆料流量对气泡生成尺寸的影响。结果表明气泡尺寸随气相流量的增大而增大,随液相流量和浆料浓度的增大而减小。     

Experimental investigation on evaporation of urea‐water‐solution droplet for SCR applications

The evaporation behavior of urea‐water‐solution (UWS) droplet was investigated for application to urea‐selective catalytic reduction (SCR) systems. A number of experiments were performed with single UWS droplet suspended on the tip of a fine quartz fiber. To cover the temperature range of real‐world diesel exhausts, droplet ambient temperature was regulated from 373 to 873 K using an electrical furnace. As a result of this study, UWS droplet revealed different evaporation characteristics depending on its ambient temperature. At high temperatures, it showed quite complicated behaviors such as bubble formation, distortion, and partial rupture after a linear D²‐law period. However, as temperature decreases, these phenomena became weak and finally disappeared. Also, droplet diminishment coefficients were extracted from transient evaporation histories for various ambient temperatures, which yields a quantitative evaluation on evaporation characteristics of UWS droplet as well as provides valuable empirical data required for modeling or simulation works on urea‐SCR systems. © 2009 American Institute of Chemical Engineers AIChE J, 2009